1. Field of the Invention
The present invention relates to power factor correction, harmonic filtering and load balancing of currents drawn from an AC utility power line by a load.
2. Description of the Prior Art
Loads connected to AC utility power lines ideally draw current that is directly in phase with the AC voltage source, is drawn equally on all phases from a polyphase source, and is of the same frequency as the line voltage. Current drawn in this fashion minimizes demand on the power source and on the distribution lines and induces no distortion of the source voltage. Many loads, however, such as motors, motor drives, computers, fluorescent lighting, and the like, do not draw power from the utility power lines in this manner.
Various prior art arrangements have been developed to correct the above-mentioned problems. For example, loads having an inductive displacement power factor can be corrected by installing power factor correction capacitors in parallel with the equipment. If sized correctly, such capacitors will supply reactive current that is directly in phase with the reactive current drawn by the load. Hence, reactive currents will circulate between the capacitors and the load thus requiring no reactive power to be supplied by the source. While effective, capacitors can only be utilized effectively if the load is constant at all times. If the load varies, however, different amounts of capacitance must be connected to the line to avoid having reactive power being drawn from the source when no current is actually needed by the load. Another problem with power factor correction capacitors is the introduction of unwanted resonance in the power lines. Switching systems, such as switching power supplies, which abruptly change their capacitance, are especially susceptible to such resonance in the power lines. Moreover, if a power factor correction capacitor is successfully installed, a different load connected to the line may degrade the performance of the system. Moreover, power factor correction capacitors do very little to correct the problems of harmonic currents and unbalanced loads.
Harmonic currents drawn by many non-linear loads that rectify the AC voltage to produce a DC supply or that phase control the AC voltage may adversely affect the power transmission system. The added current that the power transmission system must supply results in higher transmission losses that can cause large heat losses in transformers of the transmission system. Such losses require over-sized transformers. Moreover, harmonic currents can often distort the AC voltage source which can cause other equipment on the power lines to fail or perform poorly. A trap filter can be utilized to filter such harmonics. However, a separate trap filter is needed for each harmonic. A problem with trap filters, however, is the resonance caused thereby, just as in the case of the power factor correction capacitors. Still another problem is that trap filters will also try to shunt harmonics from loads other than the load to which it is attached. Hence, the trap filter's capacity is exceeded, with corresponding failure of the trap filter.
It is, therefore, an object of the present invention to provide an active harmonic filter which overcomes these problems of the prior art devices. It is an object of the present invention to provide an active harmonic filter that sources current to or draws current from the supply line at appropriate times to reduce or eliminate load induced phase shifts between the line voltage and the line current. It is a further object of the present invention to provide an active harmonic filter that reduces or eliminates line harmonics. It is yet another object of the present invention to provide an active harmonic filter that balances the currents drawn on all phases of a polyphase AC source. Still other objects of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.